When a voice call is made using a conventional analog subscriber set, only a small portion of the frequency spectrum available on the typical subscriber line is utilized. During the development of distributed data processing systems, an effort was made to utilize the existing subscriber line network to facilitate communication of the digital data used by such systems. Various types of modulator/demodulator (modem) devices have been developed to take advantage of the latent capability of the subscriber line network to support digital data communication at data rates significantly greater than that required for analog voice communication. For example, synchronous modems are commercially available which utilize the differential phase shift keyed (DPSK) modulation/demodulation technique to provide data communication at rates up to 9600 baud. However, even in the digital private automatic branch exchanges (PABXs) which support intra-exchange communication of voice information in the form of digital PCM voice data words, the response characteristics of the several analog components of the typical analog line card used therein generally prevent reliable communication at higher rates.
Several techniques have been proposed for providing higher speed digital data communication between subscribers through such PABXs. However, such proposals typically require the installation of additional pairs of subscriber lines to subscribers requiring the service and/or redesign/modification of the particular PABX to provide the capability to process the digital data words in a manner different from the digital PCM voice data words. Another problem associated with digital data communication using an existing subscriber line network is excessive power dissipation both at the subscriber set and at the PABX. In a conventional analog phone system, a subscriber set is completely powered down when the set is on hook. When the subscriber set goes off hook for a telephone set origination call, a battery from the central office powers the telephone which acts as a D.C. load. For a central office originated call, a ringing voltage is transmitted via the subscriber line which makes the subscriber phone ring. Only when the subscriber phone goes off hook does the phone draw D.C. power. In a digital PABX, the line card has electronic circuitry instead of only the transformers and relays of a conventional analog system. Others have implemented a digital PABX network with D.C. signalling and high voltage analog ringing signalling so that the components of the electronic circuitry are individually powered up or powered down in response to the telephone in the same manner as in the conventional analog system. In such systems, a subscriber set is passive and is either powered up or down depending upon whether or not the set is off hook. However, when signalling in the form of coded transmissions is used instead of direct current loop signalling or high voltage analog ringing signalling, the electronic circuitry is constantly powered up. Therefore, it is desirable to minimize the power consumption of the digital telephone set when inactive to more closely simulate the analog telephone. Existing digital telephone systems either provide power via a separate pair of subscriber lines or provide no means for reducing power consumption when in an idle state.